Stationary star-shaped antenna method for manipulating focused beamformed, shaped fields and beamsteered electromagnetic signal from subtel sedimentary stratigraphic formations deep in the earth

ABSTRACT

A method for electromagnetic geophysical surveying according to one aspect of the invention includes disposing a plurality of electromagnetic receivers in a selected pattern above an area of the Earth&#39;s subsurface to be evaluated. An electromagnetic source is repeatedly actuated proximate the electromagnetic receivers. Signals generated by the receivers, indexed in time with respect to each actuation of the at least one electromagnetic energy source, are recorded. The recorded signals are processed to generate an image corresponding to at least one point in the subsurface. The processing includes stacking recordings from each receiver for a plurality of actuations of the sources and beam steering a response of the receivers such that the at least one point is equivalent to a focal point of a response of the plurality of receivers.

BACKGROUND

1. Field of the Disclosure

The invention relates generally to the field of electromagneticevaluation formations in the earth's surface. More specifically, theinvention relates to methods for determining electromagnetic attributesin specific formations in the subsurface to a relatively high lateraland vertical resolution.

2. Description of the Related Art

Exploration for and exploitation of petroleum resources are entering anew phase wherein many methods are utilized to develop an integratedunderstanding of potential and discovered reservoir rocks. One of theoverall goals of an integrated strategy for exploration and exploitationis to reduce the risk, especially that associated with drilling in newbasins or harsh, offshore environments. Some traditional geophysicalexploration methods for oil and gas (seismic, gravity and magnetic) havebeen tuned to exploit indirect indicators of petroleum occurrences—thatis, they best delineate the structures that are potential petroleumreservoirs. Such traditional methods are not, in the first instance,direct indicators of petroleum.

Seismic methods developed over the past 60 years or so are highlyrefined and have led to a significant reduction in drilling risk to thepoint where approximately one in six exploration wells is deemedsuccessful. In the past decade, special seismic methods (4D/time lapseand attribute characterization) have been developed to further reducethe risk of drilling. The major limitation in conventional geophysicalexploration interpretation is that the physics of the methods inherentlyidentifies contrasts in macro physical properties (velocity, density andmagnetism). Thus, conventional geophysical exploration is traditionallybest suited to delineate structural traps. Such methods have beensuccessful in finding the major structures worldwide that contain oiland gas.

The primary properties of sedimentary rocks that lead to directindication of oil and gas have to do with the porosity and permeabilityof the sediments, i.e., the nature of the pore fluid, the percentage ofthe rock volume that is fluid-filled, and the migration characteristicsof the fluid. Unfortunately, while some progress has been made inutilizing the second order effects of pore fluids on seismic velocityand bulk density of the rock formations, these effects are still subtleand are traditionally subject to substantial uncertainty, especially fordeeper reservoirs as are often found in offshore basins.

In contrast to the acoustic and magnetic properties, the electricalproperties of sedimentary rocks are almost entirely determined by thevolume and nature of the pore fluids. Virtually all common rock-formingminerals in sediments are electrical resistors, for instance, quartz(SiO₂) and mica, which are often used as electrical resistors inelectronic microcircuits. Hence, because these rock-forming minerals aremost often electrical resistors, the type of sediment (carbonate,clastic rock, or salt/anhydrite) has little impact on the bulkelectrical properties of the rock and pore fluids. The electricalproperties are determined almost exclusively by the amount and nature ofthe pore fluids. Furthermore, again unlike acoustic properties that varyonly over a factor of 2 in the most extreme case (typically <10% or so),bulk electrical properties of sediments can vary by several orders ofmagnitude depending on the value of the porosity (0.1% to >20%) and thepore fluid (connate water, oil or gas). The noteworthy correlation isthat while connate water is saline to some extent and thus substantiallyelectrically conductive (compared to the rock forming minerals),petroleum fluids (i.e., oil and gas) are essentially non-conductors ofelectricity. This difference in conductivity leads to the potential toexploit this extreme property difference in a geophysical method that isa direct hydrocarbon indicator.

Those of skill in the art will recognize that significant literatureexists pertaining to the electrical properties of sediments from boththe petroleum well logging and mineral exploration fields. Empiricalrelationships have been developed that describe electrical resistivitycompared to porosity and a large body of well log correlations to guideinterpretation. These can be used to assist in determining theappropriate frequencies for any electromagnetic exploration method.

There are several electromagnetic techniques from the mineral sectorrepertoire that have recently been adapted for petroleum exploration.Prime among these is the use of towed dipole systems exploitingelectromagnetic and magnetotelluric fields. These have found favor inboth shallow water and deep marine settings. Typical electromagneticmarine surveys are extensively described in the literature and in anextensive listing of patents. The basic method involves a vessel whichtows cables connected to electrodes deployed near the sea floor. Thegeophysical support vessel generates high power signals to theelectrodes such that an alternating current of selected magnitude(magnitudes) and frequency (frequencies) flows through the sea floor andinto the geological formations below the sea floor. Receiver electrodesare deployed on the sea floor at a range of offsets from the sourceelectrodes and are coupled to a voltage measuring circuit. The voltagesmeasured at the receiver electrodes are then analyzed to infer thestructure and electrical properties of the geological formations in thesubsurface.

Another well known technique for electromagnetic surveying of geologicalformations is known in the art as transient controlled sourceelectromagnetic surveying. Typically an electric current, normallydirect current (DC), is imparted into the seafloor. At a selected time,the electric current is switched off, switched on, or has its polaritychanged (or one or more of such events occur in a coded sequence), andinduced voltages and/or magnetic fields are measured, typically withrespect to time over a selected time interval, at the Earth's surface,near the water bottom or water surface. The structure of the subsurfaceis inferred by the temporal and spatial distribution of the inducedvoltages and/or magnetic fields. These techniques are described invarious publications such as by Strack, K.-M., 1992, Exploration WithDeep Transient Electromagnetics, Elsevier, 373 pp. (reprinted 1999).

These traditional techniques for electromagnetic surveying suffer from anumber of problems. In traditional methods, low signal to noise ratiosmay make proper analysis of the electromagnetic survey difficult.Further, such methods may be deficient in that they provide a lowresolution picture of the subsurface Earth structures, again makingproper analysis problematic. Finally, such traditional methods often aredifficult to focus on particular areas of the survey, such as areas thatappear to be likely to contain petroleum bearing strata. It follows thatthere is a need to develop an electromagnetic surveying method thataddresses such issues.

SUMMARY OF THE INVENTION

A method for electromagnetic geophysical surveying according to oneaspect of the invention includes disposing a plurality ofelectromagnetic receivers in a selected pattern above an area of theEarth's subsurface to be evaluated. An electromagnetic source isrepeatedly actuated proximate the electromagnetic receivers. Signalsgenerated by the receivers, indexed in time with respect to eachactuation of the at least one electromagnetic energy source, arerecorded. The recorded signals are processed to generate an imagecorresponding to at least one point in the subsurface. The processingincludes stacking recordings from each receiver for a plurality ofactuations of the sources and beam steering a response of the receiverssuch that the at least one point is equivalent to a focal point of aresponse of the plurality of receivers.

In one embodiment of the present invention, there is provided a methodfor electromagnetic geophysical surveying, comprising:

-   -   disposing a plurality of electromagnetic receivers in a selected        pattern above an area of the Earth's subsurface to be evaluated;    -   repeatedly actuating at least one electromagnetic energy source        proximate the electromagnetic receivers;    -   recording signals generated by the receivers indexed in time        with respect to each actuation of the at least one        electromagnetic energy source; and    -   processing the recorded signals to generate an image        corresponding to at least one point in the subsurface, the        processing including stacking recordings from each receiver for        a plurality of actuations of the sources and beam steering a        response of the receivers such that the at least one point is        equivalent to a focal point of a response of the plurality of        receivers.

Other aspects and advantages of the invention will be apparent from thefollowing description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of an electromagnetic receiver array used forsubsurface imaging according to the invention.

FIG. 1A shows an electromagnetic energy source array that may be usedwith the electromagnetic receiver array shown in FIG. 1.

DESCRIPTION OF THE EMBODIMENT(S)

In one aspect of the present invention, a stationary emplacement ofelectromagnetic transmitters and receivers is placed above an area ofthe subsurface to be surveyed. The emplacement may include an array ofelectromagnetic transmitters and an array of electromagnetic receivers.In particular embodiments, the transmitters are repeatedly actuated andsignals are repeatedly detected by the receivers in the respectivearrays to coherently stack the detected signals to attain sufficientelectromagnetic signal to noise ratio. The actuation and detection ofsignals may be repeated to obtain sufficient signal to noise ratio. Suchactuation and detection typically allows for discrete beams and orshaped fields to be formed and directed deep into the subsurface

One embodiment of an electromagnetic receiver array for use with thepresent invention is shown schematically at 10 in FIG. 1. In theembodiment shown in FIG. 1, array 10 may be disposed on the bottom of abody of water 12 such as a lake or the ocean, or in an area of the landsurface below which an electromagnetic survey is to be conducted. Array10 may include individual receiver cables, such as shown in FIG. 1 as L1through L8. Cables L1 through L8 may include a plurality of spaced apartelectromagnetic receiver modules S disposed along the length of eachcable L1-L8. As would be recognized by one of ordinary skill in the artwith the benefit of this disclosure, the number, location, andorientation of individual receiver cables and electromagnetic receivermodules may be altered as needed and is not limited to that shown inFIG. 1.

Electromagnetic energy source W, which in certain embodiments may be atransmitter antenna consisting of an array of many individual sources,may be disposed either proximate center C of receiver array 10 orthroughout receiver array 10. Source W is actuated at selected times,and a time indexed record of the signals produced by each receiver ineach module S may be recorded in recording unit R for later analysis.

Cables L1-L8 may be arranged in a radial pattern as shown in FIG. 1.Cables L1-L8 in the certain embodiments may be symmetrically arrangedabout center point C of array 10 and angularly displaced from each otherby an angle of about 22½ degrees. In other embodiments, as describedabove, fewer or more cables may be used than is shown in FIG. 1. It iscontemplated that in such other examples the angular displacementbetween each of the cables will be approximately equal, however equalangular displacement between cables is not a limit on the scope of thepresent invention. The radial cable arrangement shown in FIG. 1 may beadvantageous in calculated beam steering of the spatially selectiveresponse. However, other geometric arrangements may be used that havespatially selective response according to the invention. For instance,the longitudinal spacing of receiver modules S and geometric arrangementmay be related to the maximum electromagnetic energy frequency expectedto be detected from the subsurface.

FIG. 1A shows an example of source W in more detail. Source W mayconsist of a plurality of individual antennas such as explained above,shown at W1 through W5 arranged in a small-diameter, generally circularpattern. The individual sources W1-W5 may be actuated by sourcecontroller W6, which may be in operative communication with recordingunit (R in FIG. 1) so that the signal recordings may be time indexed tothe actuation time of source W. In the present example, sourcecontroller W6 may be configured to successively, individually (or insubsets or sub-combinations) actuate each source W1-W5 at a selectedtime delay (which may be zero or any other selected time delay) afterthe actuation of the first one of sources W1-W5. The time delay orexcitation phases may be selected such that the energy output of thearray of sources W1-W5 is oriented substantially along a selecteddirection. The time delay may also be calculated from resistivityspatial distribution determined by a previously-performedelectromagnetic survey. In such examples, the directivity of source Wmay be used to further illuminate subsurface features identified duringsignal processing or otherwise.

In other examples, an array of transmitter antennas may be arrangedsubstantially the same in configuration as receiver array 10 shown inFIG. 1. The antennas in the receiver array, as well as source W or anyarray of such sources may be any one or more of the following. For thetransmitters, the antenna may be an electric or magnetic dipole. Suchdipoles may be made, respectively, by spaced apart electrode or wireloops. The dipole moment of the antennas may be vertical or horizontal.The receivers in array 10 may be antennas as explained above, or may bemagnetometers, or any combination thereof.

Electromagnetic energy may be generated by source W by passingalternating current through the antenna. Alternatively, electromagneticenergy may be generated by modulating direct current through theantenna. When using direct current, alternating current may be simulatedby such methods as switching on the direct current, switching off thedirect current, or reversing polarity of the direct current in a codedsequence.

Operating electromagnetic transmitters and receivers as explained aboveprovides electromagnetic data not previously available concerningsubsurface sedimentary structures, capturing off-specular electricalresistive scattered signals as well as the specular normal incidencereturns via the beam steered footprint or shaped field. The processingsimulates movement of the array through beam forming and beam steeringtechniques. Each image point or “spot” is formed by the electromagneticenergy backscattered in the direction of the corresponding beam withboth the source and receiver arrays being highly sensitive and capableof being steered to the point of interest in the subsurface, especiallyin complex subtle porous structures. In effect, the detected signals arerecorded and processed to generate an image corresponding to one or moreimage points in the subsurface by stacking recordings from receivers Sfor actuations of sources W and then beam steering a response ofreceivers S such that each image point is equivalent to a focal point ofreceivers S. During the electromagnetic survey technique according tocertain embodiments of the present invention, a plurality of individualfocal points may be selected through the area of the subsurface that isbeing examined.

Certain embodiments manipulate in a very stable highly coherent manner afixed array of time controlled sources; the invention allows for the useof a broad span of frequencies, for example, from the quasi-stationaryto the MHz range. These sources impart an electromagnetic field into thesubsurface formations.

Unlike the scalar amplitude measurements typically made, the method ofcertain embodiments of the present invention make measurements of thevector electromagnetic field amplitudes. Amplitudes of vector electricand/or magnetic fields are deduced through the manipulation of the beamwhich captures and quantifies in the restricted footprint of the beamvoltage differences induced in the antenna's electric and/or magneticdetectors as deployed at the surface, or on or above the sea floor. Theelectric and/or magnetic fields are induced in response to the electricfield and/or magnetic field imparted into the subsurface, and inferencesabout the spatial distribution of conductivity of the subsurface aremade from recordings of the induced electric and/or magnetic fields.High resolution gains are made by capturing much more subtle electricchanges through the sweeping of the antenna's receiver beams as steeredthrough the processing in a controlled manner using a stationary starshaped antenna array with associated adjacent energy sources with eachindividually beam steered. As, in certain embodiments, the apparatus isstationary on the Earth's surface or seafloor, temporal stacking toachieve sufficient signal to noise ratio for the beam forming andsteering is easily obtained. This is in contrast to towed systemswherein temporal stacking is generally not possible or effective. Herethe use of the term beamsteering includes both beamforming that ispossible at high frequencies wherein phase delays across the array areutilized to form and steer beams on the one hand and on the other handthe formation of shaped fields in the quasi-stationary regime. Theshaped transmitted fields are obtained by a combination of geometricdisposition of the several source current loops together with thecontrolled phasing of the currents in the loops.

In performing a method according to certain embodiments of presentinvention, it may be desirable to form as many beams as required tosweep a particular targeted geological feature in the subsurface. Theresulting images and electromagnetic attributes thus formed at a givenlocation (at a beam focusing point or area of interest in the subjectsurface which one wishes to illuminate) may have as many independentpoints as there are independent beams formed. In some embodiments, itmay be desirable to use iterative focusing strategies to enhance andimprove the focusing and thereby further the knowledge of theenvironment. One such embodiment includes illuminating specific pointsor areas in the subsurface from different angles.

The use of prior knowledge of the structure of the subsurface isimportant to successfully steer and focus the seismic sensor arraybeams. Iterative focusing strategies may be used to enhance and improvethe focusing and thereby further improve determination of the spatialdistribution of electromagnetic properties in the subsurface.

The disclosed method relies and uniquely focuses on the vectorpropagated fields; highlighting the spatial resolution and the dominanceof the propagated (wave) field, rather than the capturing of thediffused (attenuated) field as in certain other methods. Certainembodiments capture both the propagated wave field as well as thediffused attenuated field, but accentuates the wave (propagated) fieldas being the more dominant and richer in signal character.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having the benefit of thisdisclosure, will appreciate that other embodiments can be devised thatdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method for electromagnetic geophysicalsurveying, comprising: disposing a plurality of electromagneticreceivers in a selected pattern above an area of the Earth's subsurfaceto be evaluated; repeatedly actuating at least one electromagneticenergy source proximate the electromagnetic receivers; recording signalsgenerated by the receivers indexed in time with respect to eachactuation of the at least one electromagnetic energy source; andprocessing the recorded signals to generate an image corresponding to atleast one point in the subsurface, the processing including stackingrecordings from each receiver for a plurality of actuations of thesources and beam steering a response of the receivers such that the atleast one point is equivalent to a focal point of a response of theplurality of receivers.
 2. The method of claim 1 wherein the selectedpattern comprises lines of receivers radially extending from a centerpoint of an array.
 3. The method of claim 1 wherein a number ofreceivers in the selected pattern and a longitudinal spacing betweenreceivers are related to a maximum electromagnetic energy frequency tobe detected from the subsurface.
 4. The method of claim 1 furthercomprising directing energy from the electromagnetic energy sourcetoward a selected point in the subsurface.
 5. The method of claim 4wherein the directing comprises actuating each of a plurality ofindividual electromagnetic energy sources at a time causing an outputthereof to be directed substantially toward the selected point.
 6. Themethod of claim 1 wherein the beam steering comprises adding a selectedtime delay to the recording from each electromagnetic receiver.
 7. Themethod of claim 6 wherein the selected time delay is calculated fromresistivity spatial distribution determined by a previously performedelectromagnetic survey analysis.
 8. The method of claim 1 wherein eachelectromagnetic receiver comprises at least one of a vertical electricdipole, a vertical magnetic dipole, a horizontal electric dipole, ahorizontal magnetic dipole and a magnetometer.
 9. The method of claim 1wherein the electromagnetic energy source is substantially collocatedwith a center of the selected pattern.
 10. The method of claim 1 furthercomprising determining a spatial distribution of at least oneconstituent of a subsurface reservoir from the processed recordedsignals, repeating the repeated actuation, recording and processingafter a selected time period, and determining a change in the spatialdistribution of at least one constituent from the repeated processing.11. The method of claim 1 wherein the receivers and the source aresubstantially stationary during the actuating and detecting.
 12. Themethod of claim 1 wherein the actuating the energy source comprises atleast one of: passing alternating current through an antenna; passingswitched direct current through an antenna, the switching comprising atleast one of switching on, switching off, reversing polarity ofswitching in a coded sequence; and wherein the antenna comprises atleast one of vertical electric dipole, a vertical magnetic dipole, ahorizontal electric dipole and a horizontal magnetic dipole.
 13. Themethod of claim 1 wherein the electromagnetic energy source comprises anarray of electromagnetic transmitters disposed in a selected pattern.14. The method of claim 13 wherein the actuating comprises applying aselected time delay to the actuation of selected ones of thetransmitters so as to beam steer energy output of the array.
 15. Themethod of claim 13 wherein the selected pattern comprises thetransmitters being dispersed within the plurality of electromagneticreceivers.
 16. The method of claim 1 further comprising simulating beamsteering of the electromagnetic transmitters by one of selecting ageometric disposition of source current loops and controlling phasing ofelectric current in the loops.